U.S. presidential candidate Donald Trump on Friday called on his rivals to disband their Super PACs after a fundraising group backing the Republican front-runner said it would shut down over claims it inappropriately coordinated with his campaign.

"I have disavowed all Super PACs, requested the return of all donations made to said PACs, and I am calling on all presidential candidates to do the same," Trump said in a statement.

Super PACs are independent committees that can raise unlimited sums of money from corporations, unions and individuals. They have become major players in U.S. elections because they can spend massive sums of money promoting candidates, although they are not allowed to work formally with the campaigns.

Trump has complained repeatedly that Super PACs allow corporate entities and lobbyists to have undue influence. On Friday, he said because he is self-funding the bulk of his campaign to be the Republican Party's nominee for the November 2016 presidential election, he is not beholden to special interests.

While Trump disavowed Super PACs, a prominent supporter of his launched one this week. Billionaire investor Carl Icahn said Wednesday he was putting $150 million toward a new group to raise money to fight corporate "inversions" by pushing for legislation to change the tax code.

The Make America Great Again PAC said it would shut down after the Washington Post reported that one of its employees, Mike Ciletti, contacted a Trump employee to obtain information for potential donors. That would violate rules preventing coordination between campaigns and Super PACs.

Trump's campaign also asked all of the super PACs claiming to support the real estate mogul stop raising money and return funds to donors. The letters, sent on Wednesday, say the PACs do not have authorization to use Trump's likeness, according to his campaign.

Republican operatives close to Trump believe he will be hit with attack ads in the coming months, and they think calling on rivals to drop their Super PACs could pre-empt those attacks, a person close to Trump said.

Katie Packer, a Republican consultant who is critical of Trump, disagreed. The money funding Super PACs is "totally legal, disclosed dollars," she said.